Method for outer loop/weighted open loop power control apparatus employed by a base station

ABSTRACT

Outer loop/weighted open loop power control method controls transmission power levels in a spread spectrum time division duplex communication base station. The base station receives a communication including a transmitted power level and measures its received power level. Based in part on the measured power level and the transmitted power level, a path loss estimate is determined. A quality of the path loss estimate is also determined. The transmission power level for a communication from the base station is based in part on weighting the path loss estimate in response to the estimate&#39;s quality.

[0001] This application is a continuation of application Ser. No.09/533,423, filed Mar. 22, 2000; which application is incorporatedherein by reference.

BACKGROUND

[0002] This invention generally relates to spread spectrum time divisionduplex (TDD) communication systems. More particularly, the presentinvention relates to a system and method for controlling transmissionpower within TDD communication systems.

[0003]FIG. 1 depicts a wireless spread spectrum time division duplex(TDD) communication system. The system has a plurality of base stations30 ₁-30 ₇. Each base station 30 ₁ communicates with user equipment (UEs)32 ₁-32 ₃ in its operating area. Communications transmitted from a basestation 30 ₁ to a UE 32 ₁ are referred to as downlink communications andcommunications transmitted from a UE 32 ₁ to a base station 30 ₁ arereferred to as uplink communications.

[0004] In addition to communicating over different frequency spectrums,spread spectrum TDD systems carry multiple communications over the samespectrum. The multiple signals are distinguished by their respectivechip code sequences (codes). Also, to more efficiently use the spreadspectrum, TDD systems as illustrated in FIG. 2 use repeating frames 34divided into a number of time slots 36 ₁-36 _(n,), such as sixteen timeslots. In such systems, a communication is sent in selected time slots36 ₁-36 _(n) using selected codes. Accordingly, one frame 34 is capableof carrying multiple communications distinguished by both time slot andcode. The combination of a single code in a single time slot is referredto as a resource unit. Based on the bandwidth required to support acommunication, one or multiple resource units are assigned to thatcommunication.

[0005] Most TDD systems adaptively control transmission power levels. Ina TDD system, many communications may share the same time slot andspectrum. When a UE 32 ₁ or base station 30 ₁ is receiving a specificcommunication, all the other communications using the same time slot andspectrum cause interference to the specific communication. Increasingthe transmission power level of one communication degrades the signalquality of all other communications within that time slot and spectrum.However, reducing the transmission power level too far results inundesirable signal to noise ratios (SNRs) and bit error rates (BERs) atthe receivers. To maintain both the signal quality of communications andlow transmission power levels, transmission power control is used.

[0006] One approach using transmission power control in a code divisionmultiple access (CDMA) communication system is described in U.S. Pat.No. 5,056,109 (Gilhousen et al.). A transmitter sends a communication toa particular receiver. Upon reception, the received signal power ismeasured. The received signal power is compared to a desired receivedsignal power. Based on the comparison, a control bit is sent to thetransmitter either increasing or decreasing transmission power by afixed amount. Since the receiver sends a control signal to thetransmitter to control the transmitter's power level, such power controltechniques are commonly referred to as closed loop.

[0007] Under certain conditions, the performance of closed loop systemsdegrades. For instance, if communications sent between a UE and a basestation are in a highly dynamic environment, such as due to the UEmoving, such systems may not be able to adapt fast enough to compensatefor the changes. The update rate of closed loop power control in TDD istypically 100 cycles per second which is not sufficient for fast fadingchannels. Accordingly, there is a need for alternate approaches tomaintain signal quality and low transmission power levels.

SUMMARY

[0008] Outer loop/weighted open loop power control controls transmissionpower levels in a spread spectrum time division duplex communicationsystem. At a first communication station, errors are measured in areceived communication from a second communication station. Based on inpart the measured errors, an adjustment in a target level is determined.The first station transmits a communication and the target adjustment tothe second station. The second station measures the first station'scommunication's received power level. Based on in part the receivedpower level, a path loss is determined. The target level is adjusted inresponse to receiving the target adjustment. The quality of the pathloss is determined with respect to a subsequent communication to betransmitted from the second station. The second station's transmissionpower level for the subsequent communication is adjusted based on inpart the determined path loss, the determined quality and the adjustedtarget level.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 illustrates a prior art TDD system.

[0010]FIG. 2 illustrates time slots in repeating frames of a TDD system.

[0011]FIG. 3 is a flow chart of outer loop/weighted open loop powercontrol.

[0012]FIG. 4 is a diagram of components of two communication stationsusing outer loop/weighted open loop power control.

[0013]FIG. 5 is a graph of the performance of outer loop/weighted openloop, weighted open loop and closed loop power control systems.

[0014]FIG. 6 is a graph of the three systems performance in terms ofBlock Error Rate (BLER).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The preferred embodiments will be described with reference to thedrawing figures where like numerals represent like elements throughout.Outer loop/weighted open loop power control will be explained using theflow chart of FIG. 3 and the components of two simplified communicationstations 110, 112 as shown in FIG. 4. For the following discussion, thecommunication station having its transmitter's power controlled isreferred to as the transmitting station 112 and the communicationstation receiving power controlled communications is referred to as thereceiving station 110. Since outer loop/weighted open loop power controlmay be used for uplink, downlink or both types of communications, thetransmitter having its power controlled may be associated with the basestation 30 ₁, UE 32 ₁ or both. Accordingly, if both uplink and downlinkpower control are used, the receiving and transmitting station'scomponents are associated with both the base station 30 ₁ and UE 32 ₁.

[0016] The receiving station 110 receives various radio frequencysignals including communications from the transmitting station 112 usingan antenna 78, or alternately, an antenna array, step 38. The receivedsignals are passed thorough an isolator 66 to a demodulator 68 toproduce a baseband signal. The baseband signal is processed, such as bya channel estimation device 70 and a data estimation device 72, in thetime slots and with the appropriate codes assigned to the transmittingstation's communication. The channel estimation device 70 commonly usesthe training sequence component in the baseband signal to providechannel information, such as channel impulse responses. The channelinformation is used by the data estimation device 72, the interferencemeasurement device 74, and the transmit power calculation device 76. Thedata estimation device 72 recovers data from the channel by estimatingsoft symbols using the channel information.

[0017] Prior to transmission of the communication from the transmittingstation 112, the data signal of the communication is error encoded usingan error detection/correction encoder 110. The error encoding scheme istypically a circular redundancy code (CRC) followed by a forward errorcorrection encoding, although other types of error encoding schemes maybe used.

[0018] Using the soft symbols produced by the data estimation device 72,an error detection device 112 detects errors in the soft symbols. Aprocessor 111 analyzes the detected error and determines an error ratefor the received communication, step 39. Based on the error rate, theprocessor 111 determines the amount, if any, a target level, such as atarget signal to interference ration (SIR_(TARGET)), needs to be changedat the transmitting station 112, step 40. Based on the determinedamount, a target adjustment signal is generated by the target adjustmentgenerator 114. The target adjustment is subsequently sent to thetransmitting station, step 41. The target adjustment is signaled to thetransmitting station 112, such as using a dedicated or a referencechannel as shown in FIG. 4, step 41.

[0019] One technique to determine the amount of adjustment in the targetlevel uses an upper and lower threshold. If the determined error rateexceeds an upper threshold, the target level is set at an unacceptablylow level and needs to be increased. A target level adjustment signal issent indicating an increase in the target level. If the determined errorrate is below a second threshold, the target level is set at anunnecessarily high level and the target level can be decreased. Byreducing the target level, the transmitting station's power level isdecreased reducing interference to other communications using the sametime slot and spectrum. To improve performance, as soon as the errorrate exceeds the upper limit, a target adjustment is sent. As a result,high error rates are improved quickly and lower error rates are adjustedslowly, such as once per 10 seconds. If the error rate is between thethresholds, a target adjustment is not sent maintaining the same targetlevel.

[0020] Applying the above technique to a system using CRC and FECencoding follows. Each CRC block is checked for an error. Each time aframe is determined to have an error, a counter is incremented. As soonas the counter exceeds an upper threshold, such as 1.5 to 2 times thedesired block error rate (BLER), a target adjustment is sent increasingthe target level. To adjust the SIR_(TARGET) at the transmitting station112, the increase in the SIR_(TARGET) is sent (SIR_(INC)), which istypically in a range of 0.25 dB to 4 dB. If the number of CRC framesencountered exceeds a predetermined limit, such as 1000 blocks, thevalue of the counter is compared to a lower threshold, such as 0.2 to0.6 times the desired BLER. If the number of counted block errors isbelow the lower threshold, a target adjustment signal is sent decreasingthe target level, SIR_(DEC). A typical range of SIR_(DEC) is 0.25 to 4dB. The value of SIR_(DEC) may be based on SIR_(INC) and a target blockerror rate, BLER_(TARGET). The BLER_(TARGET) is based on the type ofservice. A typical range for the BLER_(TARGET) is 0.1% to 10%. Equation1 illustrates one such approach for determining SIR_(DEC).

SIR _(DEC) =SIR _(INC) ×BLER _(TARGET)/(1−BLER _(TARGET))  Equation 1

[0021] If the count is between the thresholds for the predeterminedblock limit, a target adjustment signal is not sent.

[0022] Alternately, a single threshold may be used. If the error rateexceeds the threshold, the target level is increased. If the error rateis below the threshold, the target is decreased. Additionally, thetarget level adjustment signal may have several adjustment levels, suchas from 0 dB to ±4 dB in 0.25 dB increments based on the differencebetween the determined error rate and the desired error rate.

[0023] The interference measurement device 74 of the receiving station110 determines the interference level in dB, I_(RS), within the channel,based on either the channel information, or the soft symbols generatedby the data estimation device 72, or both. Using the soft symbols andchannel information, the transmit power calculation device 76 controlsthe receiving station's transmission power level by controlling the gainof an amplifier 54.

[0024] For use in estimating the pathloss between the receiving andtransmitting stations 110, 112 and sending data, the receiving station110 sends a communication to the transmitting station 112, step 41. Thecommunication may be sent on any one of the various channels. Typically,in a TDD system, the channels used for estimating pathloss are referredto as reference channels, although other channels may be used. If thereceiving station 110 is a base station 30 ₁, the communication ispreferably sent over a downlink common channel or a common controlphysical channel (CCPCH). Data to be communicated to the transmittingstation 112 over the reference channel is referred to as referencechannel data. The reference data may include, as shown, the interferencelevel, I_(RS), multiplexed with other reference data, such as thetransmission power level, T_(RS). The interference level, I_(RS), andreference channel power level, I_(RS), may be sent in other channels,such as a signaling channel.

[0025] The reference channel data is generated by a reference channeldata generator 56. The reference data is assigned one or multipleresource units based on the communication's bandwidth requirements. Aspreading and training sequence insertion device 58 spreads thereference channel data and makes the spread reference datatime-multiplexed with a training sequence in the appropriate time slotsand codes of the assigned resource units. The resulting sequence isreferred to as a communication burst. The communication burst issubsequently amplified by an amplifier 60. The amplified communicationburst may be summed by a sum device 62 with any other communicationburst created through devices, such as a data generator 50, spreadingand training sequence insertion device 52 and amplifier 54.

[0026] The summed communication bursts are modulated by a modulator 64.The modulated signal is passed thorough an isolator 66 and radiated byan antenna 78 as shown or, alternately, through an antenna array. Theradiated signal is passed through a wireless radio channel 80 to anantenna 82 of the transmitting station 112. The type of modulation usedfor the transmitted communication can be any of those known to thoseskilled in the art, such as direct phase shift keying (DPSK) orquadrature phase shift keying (QPSK).

[0027] The antenna 82 or, alternately, antenna array of the transmittingstation 112 receives various radio frequency signals including thetarget adjustments. The received signals are passed through an isolator84 to a demodulator 86 to produce a baseband signal. The baseband signalis processed, such as by a channel estimation device 88 and a dataestimation device 90, in the time slots and with the appropriate codesassigned to the communication burst of the receiving station 110. Thechannel estimation device 88 commonly uses the training sequencecomponent in the baseband signal to provide channel information, such aschannel impulse responses. The channel information is used by the dataestimation device 90 and a power measurement device 92.

[0028] The power level of the processed communication corresponding tothe reference channel, R_(TS), is measured by the power measurementdevice 92 and sent to a pathloss estimation device 94, step 42. Both thechannel estimation device 88 and the data estimation device 90 arecapable of separating the reference channel from all other channels. Ifan automatic gain control device or amplifier is used for processing thereceived signals, the measured power level is adjusted to correct forthe gain of these devices at either the power measurement device 92 orpathloss estimation device 94. The power measurement device is acomponent of an outer loop/weighted open loop controller 100. As shownin FIG. 4, the outer loop/weighted open loop controller 100 comprisesthe power measurement device 92, pathloss estimation device 94, qualitymeasurement device 94, target update device 101, and transmit powercalculation device 98.

[0029] To determine the path loss, L, the transmitting station 112 alsorequires the communication's transmitted power level, T_(RS). Thecommunication's transmitted power level, T_(RS), may be sent along withthe communication's data or in a signaling channel. If the power level,T_(RS), is sent along with the communication's data, the data estimationdevice 90 interprets the power level and sends the interpreted powerlevel to the pathloss estimation device 94. If the receiving station 110is a base station 30 ₁, preferably the transmitted power level, T_(RS),is sent via the broadcast channel (BCH) from the base station 30 ₁. Bysubtracting the received communication's power level, R_(TS), from thesent communication's transmitted power level, T_(RS), the pathlossestimation device 94 estimates the path loss, L, between the twostations 110, 112, step 43. Additionally, a long term average of thepathloss, L₀, is updated, step 44. The long term average of thepathloss, L₀, is an average of the pathloss estimates. In certainsituations, instead of transmitting the transmitted power level, T_(RS),the receiving station 110 may transmit a reference for the transmittedpower level. In that case, the pathloss estimation device 94 providesreference levels for the pathloss, L.

[0030] Since TDD systems transmit downlink and uplink communications inthe same frequency spectrum, the conditions these communicationsexperience are similar. This phenomenon is referred to as reciprocity.Due to reciprocity, the path loss experienced for the downlink will alsobe experienced for the uplink and vice versa. By adding the estimatedpath loss to a target level, a transmission power level for acommunication from the transmitting station 112 to the receiving station110 is determined.

[0031] If a time delay exists between the estimated path loss and thetransmitted communication, the path loss experienced by the transmittedcommunication may differ from the calculated loss. In TDD wherecommunications are sent in differing time slots 36 ₁-36 _(n), the timeslot delay between received and transmitted communications may degradethe performance of an open loop power control system. To overcome thesedrawbacks, weighted open loop power control determines the quality ofthe estimated path loss using a quality measurement device 96, step 45,and weights the estimated path loss accordingly, L, and long termaverage of the pathloss, L₀.

[0032] To enhance performance further in outer loop/weighted open loop,a target level is adjusted. A processor 103 converts the soft symbolsproduced by the data estimation device 90 to bits and extracts thetarget adjustment information, such as a SIR_(TARGET) adjustment. Atarget update device 101 adjusts the target level using the targetadjustments, step 46. The target level may be a SIR_(TARGET) or a targetreceived power level at the receiving station 110.

[0033] The transmit power calculation device 98 combines the adjustedtarget level with the weighted path loss estimate, L, and long termaverage of the pathloss estimate, L₀, to determine the transmissionpower level of the transmitting station, step 47.

[0034] Data to be transmitted in a communication from the transmittingstation 112 is produced by data generator 102. The data is errordetection/correction encoded by error detection/correction encoder 110.The error encoded data is spread and time-multiplexed with a trainingsequence by the training sequence insertion device 104 in theappropriate time slots and codes of the assigned resource unitsproducing a communication burst. The spread signal is amplified by anamplifier 106 and modulated by modulator 108 to radio frequency. Thegain of the amplifier is controlled by the transmit power calculationdevice 98 to achieve the determined transmission power level. The powercontrolled communication burst is passed through the isolator 84 andradiated by the antenna 82.

[0035] The following is one outer loop/weighted open loop power controlalgorithm. The transmitting stations's transmission power level indecibels, P_(TS), is determined using Equation 2.

P _(TS) =SIR _(TARGET) +I _(RS)+α(L−L ₀)+L ₀+CONSTANT VALUE  Equation 2

[0036] The SIR_(TARGET) has an adjusted value based on the receivedtarget adjustment signals. For the downlink, the initial value ofSIR_(TARGET) is known at the transmitting station 112. For uplink powercontrol, SIR_(TARGET) is signaled from the receiving station 110 to thetransmitting station 112. Additionally, a maximum and minimum value foran adjusted SIR_(TARGET) may also be signaled. The adjusted SIR_(TARGET)is limited to the maximum and minimum values. I_(RS) is the measure ofthe interference power level at the receiving station 110.

[0037] L is the path loss estimate in decibels, T_(RS)-R_(TS), for themost recent time slot 36 ₁-36 _(n) that the path loss was estimated. L₀,the long term average of the path loss in decibels, is the runningaverage of the pathloss estimate, L. The CONSTANT VALUE is a correctionterm. The CONSTANT VALUE corrects for differences in the uplink anddownlink channels, such as to compensate for differences in uplink anddownlink gain. Additionally, the CONSTANT VALUE may provide correctionif the transmit power reference level of the receiving station istransmitted, instead of the actual transmit power, T_(RS). If thereceiving station 110 is a base station, the CONSTANT VALUE ispreferably sent via a Layer 3 message.

[0038] The weighting value, α, is a measure of the quality of theestimated path loss and is, preferably, based on the number of timeslots 36 ₁-36 _(n) between the time slot, n, of the last path lossestimate and the first time slot of the communication transmitted by thetransmitting station 112. The value of α is between zero and one.Generally, if the time difference between the time slots is small, therecent path loss estimate will be fairly accurate and α is set at avalue close to one. By contrast, if the time difference is large, thepath loss estimate may not be accurate and the long term average pathloss measurement is most likely a better estimate for the path loss.Accordingly, α is set at a value closer to one.

[0039] Equations 3 and 4 are equations for determining α.

α=1−(D−1)/(D _(max)−1)  Equation 3

α=max{1−(D−1)/(D _(max-allowed)−1),0}  Equation 4

[0040] The value, D, is the number of time slots 36 ₁-36 _(n) betweenthe time slot of the last path loss estimate and the first time slot ofthe transmitted communication which will be referred to as the time slotdelay. If the delay is one time slot, α is one. D_(max) is the maximumpossible delay. A typical value for a frame having fifteen time slots isseven. If the delay is D_(max), α is zero D_(max-allowed) is the maximumallowed time slot delay for using open loop power control. If the delayexceeds D_(max-allowed), open loop power control is effectively turnedoff by setting α=0. Using the transmit power level, P_(TS), determinedby a transmit power calculation device 98 the transmit power of thetransmitted communication is set.

[0041]FIGS. 5 and 6 compare the performance of the weighted outerloop/open loop, open loop and closed loop systems. The simulations inFIGS. 5 and 6 were performed for a slightly different version of theouter loop/weighted open loop algorithm. In this version, the target SIRis updated every block. A SIR_(TARGET) is increased if a block error wasdetected and decreased if no block error was detected. The outerloop/weighted open loop system used Equation 2. Equation 3 was used tocalculate α. The simulations compared the performance of the systemscontrolling a UE's 32 ₁ transmission power level. For the simulations,16 CRC bits were padded every block. In the simulation, each block was 4frames. A block error was declared when at least two raw bit errorsoccur over a block. The uplink communication channel is assigned onetime slot per frame. The target for the block error rate is 10%. TheSIR_(TARGET) is updated every 4 frames. The simulations address theperformance of these systems for a UE 32 ₁ traveling at 30 kilometersper hour. The simulated base station used two antenna diversity forreception with each antenna having a three finger RAKE receiver. Thesimulation approximated a realistic channel and SIR estimation based ona midamble sequence of burst type 1 field in the presence of additivewhite Gaussian noise (AWGN). The simulation used an InternationalTelecommunication Union (ITU) Pedestrian B type channel and QPSKmodulation. Interference levels were assumed to have no uncertainty.Channel coding schemes were not considered. L₀ was set at 0 db.

[0042] Graph 120 of FIG. 5 shows the performance as expected in terms ofthe required E_(S)/N_(O) for a BLER of 10⁻¹ as a function of time delaybetween the uplink time slot and the most recent downlink time slot. Thedelay is expressed by the number of time slots. E_(S) is the energy ofthe complex symbol. FIG. 5 demonstrates that, when gain/interferenceuncertainties are ignored, the performance of the combined system isalmost identical to that of weighted open loop system. The combinedsystem out performs the closed loop system for all delays.

[0043] In the presence of gain and interference uncertainties, thetransmitted power level of the open loop system is either too high ortoo low of the nominal value. In graph 122 of FIG. 6, a gain uncertaintyof −2 dB was used. FIG. 6 shows the BLER as a function of the delay. Theinitial reference SIR_(TARGET) for each system was set to itscorresponding nominal value obtained from FIG. 5, in order to achieve aBLER of 10⁻¹. FIG. 6 shows that, in the presence of gain uncertainty,both the combined and closed loop systems achieve the desired BLER. Theperformance of the weighted open loop system severely degrades.

What is claimed is:
 1. An outer loop/weighted open loop method for controlling transmission power levels at a base station employing a spread spectrum time division duplex technique having frames with time slots, comprising: a) receiving, during a first time slot, a communication including a power level; b) measuring a power level of said received communication; c) determining a path loss estimate based in part on the measured power level and the received power level; and d) setting a transmission power level for transmission during a second time slot based in part on the path loss estimate weighted by a first factor and a long term path loss estimate weighted by a second factor, wherein the first and second factors are a function of a time separation of the first and second time slots.
 2. The method of claim 1 further comprising: determining the long term path loss estimate based at least in part upon an average of path loss estimates of received communications.
 3. The method of claim 1 further comprising: determining a quality, α, of the path loss estimate based in part on a number of time slots, D, between the first and second time slot; and wherein the first factor is α and the second factor is 1−α.
 4. The method of claim 3 wherein a maximum time slot delay is D_(max) and the determined quality, α, is determined by: α=1−(D−1)/(D _(max)−1).
 5. The method of claim 3 wherein a maximum allowed time slot delay is D_(max-allowed) and the determined quality, α, is determined by: α=max{1−(D−1)/(D _(max-allowed)−1),0}.
 6. A method for controlling transmission power levels at a base station employing a spread spectrum time division duplex technique having frames with time slots for communication, comprising: a) receiving, during a first time slot, a first communication target adjustment and a first communication; b) measuring a power level of the first communication; c) determining a path loss estimate based in part on power level obtained at step (b); and d) setting a transmission power level for a transmission of a second communication during a second time slot based in part on the path loss estimate weighted by a first factor, a long term path loss estimate weighted by a second factor, and a target level adjusted by the target adjustment, wherein the first and second factors are a function of a time separation of the first and second time slots.
 7. The method of claim 6 wherein the target level is a signal to interference ratio.
 8. The method of claim 7 wherein an adjustment to the signal to interference ratio lies within maximum and minimum value limits.
 9. The method of claim 7 wherein each target adjustment is in a range of approximately 0.25 decibels to 4 decibels.
 10. A method for controlling transmission power levels at a base station employing a spread spectrum time division duplex technique having frames with time slots for communication, comprising: a) receiving a first communication; b) determining an error rate of the communication received at step (a); c) producing target adjustments as needed based in part on the error rate; and d) transmitting a second communication having a transmission power level in a first time slot and the target adjustments.
 11. The method of claim 10 wherein: a target adjustment increasing the target signal interference ratio is SIR_(INC); a target adjustment decreasing the target signal interference ratio is SIR_(DEC); a target block error rate is BLER_(TARGET); and SIR_(DEC) is determined by SIR _(DEC) =SIR _(INC) ×BLER _(TARGET)/(1−BLER _(TARGET)).
 12. The method of claim 11 wherein the BLER_(TARGET) is in the range of approximately 1% to 10%. 